1. Field of the Invention
The present invention relates to a thermal head incorporated as a printer mechanism for word processors, facsimiles or the like.
2. Description of the Related Art
A conventional thermal head incorporated as a printer mechanism for word processors or the like has a structure that a plurality of heating resistors and electrode layers are provided on an insulating substrate formed of alumina ceramics or the like with a glass-glazed layer in between, and the heating resistors are coated with a protective layer with a thickness of approximately several microns, and functions as a thermal head. A predetermined electric power is applied to the heating resistors through the electrode layers based on external image data to individually and selectively cause the heating resistors to generate Joule heat, and the generated heat is transmitted to a recording medium such as thermosensitive paper to form a predetermined print image on the recording medium.
The protective layer is provided for protecting the heating resistors and the like from abrasion due to contact of the sliding recording medium and corrosion due to contact of moisture and the like in the atmosphere. The protective layer is formed of an inorganic material having excellent abrasion resistance such as silicon nitride (Si3N4), silicon carbide (SiC) or tantalum oxide (Ta2O5).
However, in the conventional thermal head, in the case where the protective layer is formed of silicon nitride or tantalum oxide, since the specific resistances of these materials are high (e.g., silicon nitride: 1xc3x971012xcexa9xc2x7 cm; tantalum oxide: 1xc3x971014xcexa9xc2x7 cm), when printing is performed by forcing the recording medium into slidable contact with the protective layer on the heating resistors, static buildup occurs accumulated on the surface of the protective layer as the recording medium slides thereon, and when the static buildup reaches a predetermined amount, discharge occurs between the surface of the protective layer and the heating resistors, so that a dielectric breakdown of the protective layer is caused. In this case, not only the function as the protective layer is lost but also a large amount of current momentarily flows through the heating resistors with the dielectric breakdown, so that the heating resistors are burnt.
In the case where the protective layer of the thermal head is formed of a typical silicon carbide consisting of 50% carbon and 50% silicon, since the specific resistance of the silicon carbide is 8xc3x97107xcexa9xc2x7 cm which is smaller than those of silicon nitride and tantalum oxide, when static electricity is applied to the surface of the protective layer, the static charge is dissipated to some extent to decrease the occurrence of the dielectric breakdown. However, when the recording medium is formed of a material low in moisture absorbency such as plastic, an extremely large amount of static electricity is applied to the surface of the protective layer. Consequently, a dielectric breakdown is often caused like in the case of silicon nitride and tantalum oxide.
To solve the above-mentioned drawbacks, it has been proposed to cover the protective layer with a conductive layer formed of chromium (Cr) or the like so that the static charge is excellently dissipated into the entire area of the conductive layer.
However, when the protective layer of the thermal head is covered with the conductive layer, high thermal stress is applied between the protective layer and the conductive layer because the thermal expansion coefficients of the inorganic material of the protective layer and the metal such as chromium of the conductive layer are largely different. For this reason, when the recording medium slides on the surface of the conductive layer, the conductive layer easily comes off the surface of the protective layer because of the thermal stress and the slide of the recording medium, so that the charge dissipating function is lost.
The invention is made in view of the above-mentioned drawbacks, and a thermal head of the invention comprises an insulating substrate, heating resistors provided on the insulating substrate, and a protective layer containing carbon and silicon, the heating resistors being coated with the protective layer, wherein the protective layer contains 65 atm % to 90 atm % carbon and carbon-to-carbon bonds of the protective layer include 95.0% or more covalent bonds related to an sp2 hybrid orbital.
Moreover, in the invention it is preferable that the protective layer has a specific resistance of 2xc3x97104xcexa9xc2x7 cm to 1xc3x97107xcexa9xc2x7 cm.
Further, in the invention it is preferable that the protective layer contains 70 atm % or more carbon.
Further, in the invention it is preferable that Vickers hardness Hv of the protective layer is 1700 to 2300.
Further, in the invention it is preferable that the protective layer is formed so as to have a thickness of 1.5 xcexcm to 4.0 xcexcm.
Further, in the invention it is preferable that the thermal head comprises a barrier layer formed of silicon nitride, silicon oxide or SIALON (Sixe2x80x94Alxe2x80x94Oxe2x80x94N), the barrier layer being interposed between the heating resistors and the protective layer.
Further, in the invention it is preferable that the respective heating resistors and the barrier layer contain 20 atm % to 60 atm % silicon.
A thermal head of the invetion comprises an insulating substrate; heating resistors provided on the insulating substrate; and a protective layer containing carbon and silicon, the heating resistors being coated with the protective layer, wherein the protective layer contains 65 atm % to 90 atm % carbon and carbon-to-carbon bonds of the protective layer include 95.0% or more covalent bonds related to an sp2 hybrid orbital, and wherein the heating resistors are formed of an electrical resistance material selected from the group consisting of TaSiO, TaSiNO, TiSiO, TiSiCO, NbSiO and TiSiNi.
According to the thermal head of the invention, since the heating resistors are coated with the protective layer containing carbon and silicon, the carbon content of the protective layer is 65 atm % to 90 atm % and 95.0% or more of the carbon-to-carbon bonds (Cxe2x80x94C bonds) are covalent bonds related to an sp2 hybrid orbital, the protective layer is provided with moderate conductivity and a sufficient insulation property for preventing a short circuit between electrode layers, so that when printing is performed with use of a recording medium of a material low in moisture absorbency such as plastic, even if an extremely large amount of static electricity is applied to the surface of the protective layer because of the slide of the recording medium, the static charge is excellently dissipated into the entire area of the protective layer to effectively prevent a dielectric breakdown of the protective layer. Consequently, the protective layer excellently functions over a long term, so that burnout of the heating resistors due to the dielectric breakdown of the protective layer never occurs on the heating resistors.
Moreover, according to the thermal head of the invention, since the carbon content of the protective layer is 70 atm % or more, the thermochemical stability of the protective layer can be improved, so that even if the temperature of the protective layer is somewhat high, for example, when the thermal head is used, the protective layer can be effectively prevented from being partly lost because of silicon in the protective layer chemically reacting with hydroxyl radicals (OH radicals) in the recording medium. Consequently, it is possible to coat the heating resistors with the protective layer in an excellent condition over a long term.
Further, according to the thermal head of the invention, since the Vickers hardness Hv of the protective layer is in a range of 1700 to 2300, the protective layer excellently functions over a long term. Moreover, in this case, since the protective layer itself dissipates static charge, as long as the protective layer exists, the static charge due to the slide of the recording medium can be dissipated.
Further, according to the thermal head of the invention, since the barrier layer formed of silicon nitride, silicon oxide or SIALON is interposed between the heating resistors and the protective layer, the protective layer can have extremely high specific resistance, so that a problem can be effectively avoided such that when an extremely large amount of static electricity is applied to the surface of the protective layer because of the slide of the recording medium, part of the static charge flows into the heating resistors to vary the amount of the current passed through the heating resistors. In addition, corrosion due to contact of oxygen, moisture etc. in the atmosphere is more reliably prevented by excellently isolating the heating resistors from the atmosphere, so that the corrosion resistance can be further improved.
Further, according to the thermal head of the invention, since the silicon contents of the heating resistors and the barrier layer are 20 atm % to 60 atm %, the heating resistors, the barrier layer and the protective layer contain substantially the same amount of silicon, so that conformities between the heating resistors and the barrier layer and between the barrier layer and the protective layer are excellent. Consequently, the adhesions of the barrier layer and the protective layer to the substrate are improved.